Display devices that display images by modulating discretely arranged matrix pixels by light reflection, transmission, or light emission as in a liquid crystal, a plasma, an EL (electro-luminescence), a DMD (digital mirror device) or the like, are used for various image display devices such as projectors, and monitors for use with computers, other than thin screen television sets or rear-projection television sets. In recent years, owing to technological leaps in high-definition television broadcast and processing speed of computer systems, as high definition imaging technology advances, higher intensity technology also rapidly progresses on account of improvement of the associated devices. Along with the high intensity display devices, a quantization noise that occurs when digitized signal images are displayed has become noticeable.
When analog image signals are received, the quantization noise is generally reduced by increasing the quantization number of an analog-to-digital (A/D) converter (for instance, the number of bits increases from 8 bits to 10 bits); however, now that high definition images have become commonplace, the increase of the quantization number of the A/D converter leads to significant cost increase. Moreover, due to a signaling protocol between a sending party and a receiving party the digital transmission system does not allow the quantization number to be one-sidedly varied. Likewise, since DVDs, or network digital contents also have the predetermined quantization number, the increase of the quantization number of the contents per se is hard to increase because a new specification needs to be established.
Furthermore, in signal processing of the television and the like, an expansion of the dynamic range of an image signal received to display impact-filled images and a conversion of the images into high contrast ones by changing gray level to the intensity value, are carried out. The expansion process of the dynamic range and the change of gray level to the intensity value results in the occurrence of banding (a noticeable jump in the gray levels between adjacent regions—hereinafter referred to as “tone jump”) while displaying high contrast images.
In such situations, there is disclosed a method in which tone jump is reduced by carrying out the following: a combination of gray levels in a case where the tone jump is caused with image processing is searched; a region having the gray level combination is identified from among image signals; and in the region of the image, the tone jump is reduced by implementing the processing of grayscale interpolation to neighboring pixels where the tone jump is caused. (See, for instance, Japanese Unexamined Patent Publication H10-84481 (paragraph 0051 through 0055, FIG. 9.))
However, in the above-described processing, a combination of gray levels in which the tone jump is caused has been preliminary searched; a region adjacent to the combination of gray levels in which the tone jump is caused has been detected from among image signals; and the processing of grayscale interpolation has been implemented in the periphery of the edge of the detected region. A quantization noise, which has a similar characteristic as the tone jump, can also be reduced by the above-described processing. However, a problem has been that if such a processing method is used to reduce the quantization noise and tone jump, image signals that are delivered temporally continuously cannot be processed timely, thus causing the received image signals not to be processed in real time.